By "diverse," do they mean ethnically diverse?
I feel like, either retain the 50s setting and the original cast in all their whiteness, or write an updated version if you want a more ethnically diverse cast (or just use the "kids on an adventure" theme to write completely different adventure stories?)
1950s was, quite honestly, overwhelmingly white and it certain wasn't color-blind; if you have a bunch of multi-ethnicity kids wandering around the British countryside c. 1950, people would have noticed and commented and said things. If non-white characters are handled in this kind of color-blind way, it pretty much amounts to lying about the past.